Video adaptor cards that can drive two monitors have been available for PCs for some time. Also, a PC can includes two or more video adaptor cards, each of which is able to drive at least one monitor. However, a problem arises when a user attempts to disconnect or connect an analog video graphics array (VGA) monitor to a PC without rebooting the PC after such a change is made. There is no reliable method currently available for a computing device to universally and reliably detect when an analog monitor is connected or disconnected from the video graphics adaptor of the computing device. Detecting when such a monitor is disconnected from a “running” computer system is especially important, since the change can occur when the user is switching from a multiple monitor display environment to a single monitor environment, or when a secondary display is connected/disconnected to/from a PC or laptop computer, such as a display projector that is being used during a presentation. Because the monitor disconnect is not detected by the operating system, the user can be left with a “phantom” monitor situation where cursors and window panes are treated by the operating system as if they were continuing to be displayed on what is then a non-connected monitor. Problems can also arise when a laptop is connected/disconnected to/from a docking station that is coupled to an external analog monitor, or its video output terminal is directly connected to or disconnected from an external analog monitor as a result of the computing system's inability to detect whether the external analog monitor is connected to the system.
Although some newer Video Electronics Standards Association (VESA) monitors integrate Extended Display Identification Data (EDID) signals onto a VGA connector, not all VGA monitors (especially older monitors) or conference room VGA projectors include EDID information. EDID is a data structure provided by a monitor to describe its capabilities to a video graphics adaptor card, enabling a PC to know what kind of monitor is connected. EDID is defined by a standard published by VESA. The information provided to a PC from an EDID capable monitor includes a manufacturer name, a product type, phosphor or filter type, timings supported by the display, display size, luminance data and (for digital displays only) pixel mapping data. Although a PC could detect when signals conveying EDID are connected or disconnected to a VGA port on a video graphics adaptor card, the fact that many monitors do not provide the signals conveying EDID information means that this method is not usable to reliably detect the hot connect or disconnect of any analog monitor to a video graphics adaptor card.
Typically, the prior art has relied on analog monitor attach detection during computer boot-up. In this case, the connected state of the one or more monitors is detected at boot-up. Also, if no monitor is connected to a video adaptor on a PC when the PC is booted up, the PC may produce an audible beeping code as an error signal indicating that no monitor is connected, and preventing the PC from completing the boot-up. However, once booted-up, there is no provision in this prior art detection scheme to “hot detect” that an analog monitor has been disconnected from the PC, or to “hot detect” when, for example, an analog monitor has been connected. While detection of digital monitors is provided by new monitor standards (EDID/DDC or DVI), which are able to detect when a digital monitor is connected to or disconnected from a DVI drive port, there is no provision for providing the same function in regard to an analog video graphic adaptor (VGA) monitor. Thus, it would be desirable to develop a method that will detect the connection or disconnection of any analog monitor to a computing system via the ubiquitous 15-pin VGA connector that has been used on computing systems since about 1987. The detection system should not require any modification of an analog monitor, i.e., should be transparent to the monitor and the user and should detect the hot connect or disconnect of any such analog VGA monitor.